As a rule, manually actuated roll-up window shades are somewhat simpler in construction than electrically operated roll-up window shades. For example, electrically operated roll-up window shades require a drive motor, which increases costs. If a roll-up window shade is located in the effective arms reach of the driver, a manually activated roll-up window shade can be used easily. A preferred type of roll-up window shades that can be actuated manually is a roll-up sunroof shade. Roll-up sunroof shades are generally designed such that they can remain in any arbitrary intermediate position.
The use of a wind-up shaft that is biased in a wind-up direction of the shade element with the help of a spring motor is the simplest form of such a roll-up shade. The edge of the shade element away from the wind-up shaft is connected to a pull rod that is guided in side guide rails and can be operated by hand. Braking devices on the pull rod help ensure that the pull rod remains in each desired position and is not moved in the direction of the wind-up shaft by the spring motor. For this purpose, the braking power must be relatively strong because the spring motor provides a relatively strong retraction force when it is at the end of its activation path, (i.e. the shade is completely extended) even when a spring motor having a flat characteristic line is used.
Moreover, with such an arrangement, the tensile stress in the shade element differs very widely depending on whether the shade is pulled out or largely retracted. Another disadvantage of such an arrangement is that the activation force that must be applied on the shade element is significantly greater when the shade is being extended than when being retracted because of the braking devices and the retracting force applied by the spring motor.
To obtain a more constant activation force, a cord can be attached to each end of the pull rod so that it extends from the pull rod to the distal end of the guide rail out of view of the wind-up shaft as shown in DE 10 2004 038 756 A1. At the distal end of the guide rail the tension cord turns around and extends back to the wind-up shaft. A pulley, on which the cable is wound, is arranged coaxial to the wind-up shaft for each tension cable. A spring compensating device is provided between the pulley and the wind-up shaft. The spring compensating device is necessary to compensate for the difference in length produced when the shade element is unwound from the wind-up shaft while on the other side the tension cord is wound onto the pulley with constant effective radius. This arrangement has proven to work rather well, but the tension cord is not easy to mount.